The present invention relates to the material processing arts. It particularly relates to the coating or annealing of metal strips, such as carbon steel strips, in a controlled atmosphere using combustion furnace heating augmented by induction heating, and will be described with particular reference thereto. However, the invention will also find application in other material manufacturing and surface treatment processes that take place in a chamber inside which induction heating coils are advantageously disposed for controlled heating.
Many steel processing steps such as annealing, coating, galvannealing, and the like require thermal cycling of the subject steel, which steel is typically in the form of a strip, bar, tube, pipe, or other shape. The prior art includes combustion furnaces with augmentation by induction heating to effectuate the proper thermal cycle. Augmentation by induction heating increases furnace capacity.
In order to be effective, the induction heating coils should closely coupled to the subject material, and preferably to the subject metal. However, the coils are typically disposed outside the furnace. This arrangement reduces inductive energy transfer to the steel due to the spatial separation. Improved coupling can be obtained by modifying the shape of the furnace shell, e.g. by necking down the furnace in the vicinity of the induction coils.
Alternatively, the coils can be placed inside the furnace. This arrangement has the advantage of close coupling between the coils and the steel strip or bar. However, placing the coils inside the furnace increases the likelihood of coil damage and failure due to impingement upon the closest structure in the confined interior space. Furthermore, the prior art methods for replacement of coils disposed inside the furnace typically involve a complete shutdown of furnace operations and removal of the damaged coil or coils, using a crane or other heavy machinery to access the furnace interior and remove the entire induction coil.
Generally, the root cause of failure in a furnace employing induction heating is impingement by the steel strip on the closest structure. In the case of induction coils arranged outside the furnace, this corresponds to impingement of the strip on the insulation at the necked-down region of the furnace. This insulation is preferably thin to improve heater coupling with the steel strip or bar. However, the insulation is also preferably thick enough to ensure adequate furnace insulation. Furthermore, the insulation should be as far away as possible from the strip to avoid contact therewith. Thus, compromises are made with respect to the insulation thickness and the size of the furnace opening in the necked-down region, and these compromises in turn limit the size of the steel strip or bar that can be accommodated by the furnace.
External placement of the coils advantageously enables coil replacement without shutting down the furnace. However, in this arrangement the closest structure to the strip is the insulation rather than the coil, and so insulation failure is more likely than coil failure. Insulation repair or replacement usually cannot be performed without a complete furnace shutdown.
Locating the coil inside the furnace eliminates the need for necking-down the furnace shell and improves inductive coupling with the steel strip. However, with this arrangement the coils are the closest structure to the steel strip, and therefore the coils are the most likely element to fail. The coils can be encased in a refractory material to reduce the probability of damage thereto. However, the refractory introduces the disadvantages of less efficient coupling due to the intervening refractory material, and larger coil size. The increased coil size due to the additional refractory coating is particularly disadvantageous due to the limited space available inside the furnace. Also, in the event that the strip contacts the refractory material, contamination of the steel strip becomes a major quality issue. In the case of coil or refractory failure, the furnace usually must be completely shut down and the entire coil removed.
Locating a bare coil inside the furnace enables the maximum electrical efficiency along with the maximum trip movement without contact with any structures, e.g. the coils. However, coil repair of such prior art induction heating systems again typically involves a complete furnace shutdown and removal of the entire coil using a crane or other heavy machinery.
The present invention contemplates an improved induction coil which overcomes the aforementioned limitations and others.